In a conventional vehicle engine, a cylinder compression ratio (CR) is fixed, with a piston moving between a consistent top-dead-center (TDC) and bottom-dead-center (BDC) during each combustion cycle. If the CR is set at a low ratio to deliver maximum power during engine operation, the low CR may result in undesirable combustion of excess fuel during light engine loads and speeds. Conversely, if the CR is set at a high ratio to prioritize fuel efficiency, a power output of the engine may be degraded when increased torque is requested.
To mitigate the above issues, an engine may be adapted as variable compression ratio (VCR) engine and equipped with various mechanisms to alter (e.g., mechanically) a volumetric ratio between the piston TDC and BDC. Thus the CR may be adjusted as engine operating conditions change. As a non-limiting example, a VCR engine may be configured with a mechanical piston displacement changing device (e.g., an eccentric) that moves the piston closer to or further from the cylinder head, thereby changing the size of the combustion chambers. Still other engines may mechanically alter a cylinder head volume.
VCR engines may allow increased fuel efficiency over conventional engine systems with fixed CRs. However, when the CR is adjusted in a VCR engine, maintaining the mechanism in a position to sustain operation at a given CR may include operating an electric motor to brace the mechanism against forces arising from combustion and inertia that would otherwise change the position of the VCR device. Although the VCR engine may provide increases in fuel economy of 3-4%, activation of the motor to maintain the CR may at least partially offset the fuel economy benefits.
Attempts to address the additional fuel consumed by the electric motor while maintaining the CR may include configuring the VCR mechanism with a device to hold the desired CR without use of the motor. One example approach is shown by Aoyama et al. in Japanese Patent Application No. JP 2003322036. Therein, a VCR mechanism comprises multiple linkages connecting a piston and crankshaft to a rotatable control shaft. Rotation of the control shaft is controlled by an electric motor and the rotation varies the CR. When a command to alter the CR is detected, the electric motor adjusts the control shaft until a desired CR is attained. A hydraulic retention device is actuated to hold the position of the control shaft based on oil pressure in the device. The electric motor may be deactivated until the CR is to be varied, thus decreasing energy directed towards operating the motor.
However, the inventors herein have recognized potential issues with such systems. The CR of the engine is maintained entirely based on hydraulic pressure. During engine operation, the piston motion exerts force directly on the control shaft, which may transmitted to the hydraulic retention device, thereby requiring the device to absorb vibrations and fluctuations in pressure and temperature in order to retain the position of the control shaft. Absorption of such forces may accelerate degradation of the VCR mechanism components. Furthermore, the hydraulic retention device may be prone to slippage when fluctuations in oil pressure occur. Slipping of the retention device may lead to undesirable deviation of the CR from the target CR.
In one example, the issues described above may be addressed by a variable compression ratio (VCR) mechanism comprising an S-link including a plurality of apertures and coupled to a control shaft and attached to an electric motor, a locking pin configured to alternate between a first position and a second position, the locking pin inserted through an aperture of the plurality of apertures when in the first position, and a hydraulic device configured to selectively maintain the locking pin in the first position. In this way, the CR may be varied by an electric actuator and held in place without consuming energy via a hydraulically-actuated mechanical pin.
As one example, the VCR mechanism includes an S-shaped link, configured with a plurality of apertures, coupled to a control shaft of the VCR engine. Rotation of the shaft is achieved by an electric actuator, the electric actuator adjusting a position of the link relative to the control shaft, thereby rotating the control shaft and altering piston heights within combustion chambers of the VCR engine. Altering the piston heights results in adjustment of the CR. The position of the link is sustained by a locking pin that is inserted in an aperture of the plurality of apertures. Movement of the locking pin is controlled by a two-position valve that varies hydraulic pressure within the VCR mechanism to alter a position of the locking pin. In this way, the VCR mechanism may maintain a piston of the VCR engine at a target CR without relying on an active electric motor.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.